The qualitative and quantitative features of the host parasite relationship in Trypanosoma cruzi infected mice has been studied. Silica treated resistant mice have been utilized in studying the in vivo role of macrophage in T. cruzi infection. The study reaffirms the importance of macrophages in the resistance against T. cruzi. We initiated a biochemical study of the parasympathetic nervous system of resistant and susceptible mice infected with T. cruzi. Choline acetyltransferase was depleted in the hearts of susceptible infected mice whereas no depletion was found in resistant mice. Immunosuppression was examined in mouse strains resistant or susceptible to the Brazil strain of T. cruzi. Spleen cells from infected resistant and susceptible mice had a reduced response to Con A and LPS. However, the in vivo primary antibody response to SRBC was inhibited in infected susceptible mice but an augmented response was demonstrated in resistant mice.